In a liquid crystal display device using a LED backlight, methods of controlling brightness of an LED are roughly divided into two types of methods, that is, a pulse current dimming scheme and a constant current dimming scheme. In the pulse current dimming scheme (hereinafter, “PWM dimming”), visual brightness is controlled by changing a percentage of an ON period and an OFF period of an electric current, that is, a duty ratio while maintaining a current value of an electric current flowing through an LED to be constant. In the constant current scheme (hereinafter, “constant current dimming”), visual brightness is controlled by changing a current value of an electric current flowing an LED.
In the PWM dimming, switching control of the ON period is consequential, and thus an accurate adjustment is possible, and an LED current is constant during the ON period. Further, in the PWM dimming, since there is no change in characteristics of the LED at the time of lighting, it is easy to control chromaticity or the like, and the PWM dimming is widely used as a current dimming scheme. However, in the PWM dimming, a dimming ratio is restricted according to a rising/falling time of a driving current, and thus the dimming ratio may not be sufficiently obtained. As a solution to this problem, there is a technique of increasing a dimming range by simultaneously controlling a pulse and an electric current such that a driving current value is decreased while decreasing the duty ratio of the PWM.
In addition, the PWM dimming has a problem in that flickering is seen by some people. Further, in the PWM dimming, as the current value of the electric current flowing through the LED increases (luminance increases), a current change at the time of ON/OFF increases, and thus ripples are likely to overlap at a power source circuit side. Thus, in the PWM dimming, there is a problem in that a ringing sound is likely to be generated in a circuit member such as a capacitor or a coil. For this reason, there are recently cases where, in order to prevent flickering of the LED or an ON/OFF change of the LED current, a constant current dimming scheme of increasing only the driving current without performing pulse width modulation of the driving current and controlling luminance of the LED is used. In the constant current dimming scheme, the voltage/current ripples, the ringing sound, and visibility are improved, but since an electric current is controlled in an analogue manner, an electric current error has directly influence on luminance characteristics of the LED. Thus, it is harder to perform control at a low luminance side (a low current value) than in the PWM dimming, and there is a problem in that luminance is likely to be uneven.
In the case of dimming an LED backlight with a plurality of parallel LED circuits, it is necessary to simultaneously perform dimming of a plurality of current sources, that is, a plurality of constant current circuits. If the dimming is performed according to the constant current dimming scheme, due to individual differences of the constant current circuits, there is a difference in the driving current value, and thus the respective LED columns differ in luminance. Thus, the constant current dimming scheme has a problem in that the in-plane luminance of the entire LED backlight becomes uneven.
For example, when the LED backlight is driven using two constant current circuits A and B driving 100 mA at dimming of 100%, due to an error between the circuits, if the driving current of 101 mA and the driving current of 99 mA flow through the circuits A and B, respectively, at dimming of 100%, an error between the circuits is 2 mA, and a luminance difference with respect to the driving current is about 2%. However, when 11 mA and 9 mA flows through the circuits A and B, respectively, at dimming of 10%, the luminance difference with respect to the driving current is close to 20% even there is the same error, that is, 2 mA. In other words, when the difference (error) in the current between the constant current circuits is almost constant regardless of the dimming ratio, as the driving current value is decreased at the time of low dimming (at the time of a low current), the ratio of the current difference of the driving current value between the constant current circuits with respect to the driving current value is increased, and the current difference is likely to be particularly remarkably seen as a luminance difference, leading to luminance unevenness. Since an error of a current value of a neighboring LED is often viewed as unevenness even at 10%, depending on an in-plane luminance design of the entire backlight, it is difficult to use low dimming of less than 20% in the constant current circuit having the error of 2 mA.
As the solutions to this problem, there are a technique of averaging a difference in luminance by alternately switching a current source and an LED through a switch and performing driving in a time division manner and a technique of removing a difference between current sources by driving a plurality of LEDs in a time division manner with respect to one current source.
In the former, it is solved by alternately switching a current source and an LED when a parallel number is 2, but as the parallel number increases, control and a combination of alternate driving become more complicated, and thus a circuit size is likely to increase significantly. In the latter, the ON period of the electric current for each of the LEDs that are connected in parallel is necessarily the reciprocal of the parallel number, and a maximum luminance of the backlight is commonly equal to or less than half of the luminance when the LED is constantly turned on, and thus it is difficult to use luminous efficiency sufficiently. Further, the known example is under the assumption of a method of performing dimming according to a time interval or a pulse width and thus deals with neither a problem nor a solution at the time of low dimming in a dimming scheme based on an increase in an electric current.
As a solution to a variation in brightness at the time of low luminance dimming in the constant current dimming, there is a technique of causing an electric current of a pulse form to flow through an LED at the time of low luminance and performing dimming by changing an average value (that is, a duty ratio or a frequency) of pulse waveforms. This known example is an effective method in securing linearity and reproducibility of dimming-luminance characteristics of an LED. However, the method of performing dimming based on the duty ratio or the frequency is the same scheme as the conventional PWM dimming, and still has the problem such as a noise, a ringing sound, flicking, or ripples. Further, there is a problem in that since the LED current value (peak value) is constant when dimming is performed based on a pulse average value, the differences in the electric current at the time of low luminance is not improved when a plurality of parallel LEDs are driven by a plurality of constant current circuits.